1. Field of the Invention
The present invention relates to a disk drive having reduced data access time in performing a track seeking operation, and more particularly, a disk drive apparatus and method for selecting a seek profile based on a read or write command for reducing data access time in the disk drive.
2. Description of the Prior Art
One measure of performance of a hard disk drive is its data access time. Data is stored on data storage disks in concentric data "tracks", which are divided into "wedge-like" sectors. Servo information is recorded in the radially continuous narrow wedges between sectors, including track number, sector number and tracking information. To maximize performance of a disk drive, it is desirable to minimize data access time, including the time required to move the disk drive actuator from a current data track to the "target" data track.
The process of moving a head from a current track position to a desired or target track position is known as a "seek". The disk drive includes a servo system that is utilized both to seek to a selected target track and thereafter follow the target track on the data storage disk. A seek to a selected target track is commonly made in accordance with a profile of command effort to the actuator for a respective seek distance, which is stored in memory and accessible by the servo system controller. In known embodiments, the same seek profile is utilized for a given distance for execution of read and write commands.
The seek profile can be described in terms of acceleration, velocity, or position. A seek profile can include three components; an acceleration profile, an optional coast profile, and a deceleration profile. The acceleration profile (typically, but not necessarily set to the maximum acceleration permitted by the hardware) involves the initial portion of the seek when the actuator is gaining speed. The coast profile, which may or may not be used, holds the velocity substantially constant at some predetermined velocity. The deceleration profile ends with both acceleration and velocity close to zero as the head approaches the target track.
In FIGS. 1-4, sample idealized acceleration and velocity seek profiles for two prior art seeks for a given distance are shown. Referring to FIGS. 1 and 2, acceleration and velocity profiles graphically illustrate a first prior art seek operation. In FIG. 1, the actuator is commanded to accelerate at time T0. This acceleration is maintained until the velocity of the actuator reaches a peak value VCREST, shown in FIG. 2. This occurs at time Tswitch. The actuator is then commanded to decelerate, until time Tend at which time the deceleration and velocity are brought back to zero, and the head is positioned at the target track.
Referring to FIGS. 3 and 4, acceleration and velocity profiles graphically illustrate another prior art seek operation in which a coast period is used. As illustrated, at time T0 the actuator is commanded to accelerate. This acceleration is held until the actuator reaches maximum velocity VCREST at time Tm, where Tm is the length of time required to reach maximum velocity. In this example, the maximum velocity VCREST is held (in a "coast" mode) until time Tn at which time the actuator is commanded to decelerate so that the velocity decreases to zero at time Tend.
The velocity profiles illustrated in FIGS. 2 and 4 are idealized profiles in which the head velocity reaches zero at time Tn. It is understood in the art that many variables, including resonant modes of the actuator and stored energy in the actuator, prevent a precise correction of actuator velocity which would result in the head landing exactly on track at the conclusion of the seek. These variables may cause the head to overshoot the target track. In any event, a settling period is required to position the head within an acceptable range of the target track center. The settling period adds to the total time of the seek operation and may be extended or reduced according to the shape of the applied seek profile (e.g., a more aggressive, or faster profile will cause larger and longer residual vibrations).
Disk drives are capable of storing large amounts of data in part due to a corresponding high density of data tracks on the disk. As such, the heads must be closely aligned with the target track for reading and/or writing of data without error. Off track thresholds or windows are defined about the tracks beyond which reading or writing will not take place. These can be termed a read settle window and a write settle window.
Read settle and write settle window criteria are primarily defined by head width parameters. The required size of the head corresponds to the density of data tracks on the disk. Proximity of the head to the target track during a write operation is more critical than a read operation due to the potential of corrupting data in an adjacent track. As such, the required read settle window for execution of a read command is typically much wider than a write settle window for execution of a write command. Execution of a read command outside of the read settle window may result in a read error. The read error may cause a "retry", which can result in the disk being rotated through a full revolution before the location to which the data is to be read comes into alignment with the read head. Execution of a write operation outside of the write settle window can result in data being partially written to an adjacent track, which corrupts previously stored data therein. Because read and write errors can greatly increase the time required to access a disk drive target track for execution of a read or write operation, the servo controller tries to ensure that the head is properly aligned with the target track and the vibrations owing to a seek have reached acceptable limits (i.e., the head is staying within the window) before execution of a read command or write command.
Known systems utilize the same seek profile to position the head for execution of a read or write operation. Such profiles do not provide optimal disk drive data access times, as they may be overly conservative on read seeks which do not require as strict head positioning and alignment criteria as the execution of a write command, or may err on the aggressive side and cause unnecessarily long write settles.